Contemporary multi-mode multi-standard wireless communication systems require high performance radio receivers (a.k.a. receivers). The multi-mode receiver must provide adequate signal-to-noise (SNR) performance for weak signals to achieve maximum sensitivity performance. Additionally, the multi-mode receiver must linearly handle signal and interference levels over a wide dynamic range with minimal distortion. That is, high linearity performance is needed. Distortion is caused by, for example, intermodulation and gain compression. Higher linearity results in reduced intermodulation levels and gain compression. Consequently, low noise, high gain performance is also needed. Typically, receiver design techniques which simultaneously provide both high linearity and low noise are difficult to achieve and subject to design compromises. In addition, many wireless communications devices which are also mobile rely on rechargeable batteries as their energy source, so low dc current consumption is also required to extend battery life.
One important constituent of a high performance receiver is a low noise amplifier (LNA). The LNA is the main determinant of the overall noise performance of the receiver. In other words, the characteristics of the LNA (such as high linearity and low noise) will dominate the overall receiver performance. Generally, the LNA is placed at the front-end of the receiver, near the receive antenna interface, to minimize radio frequency (RF) losses between the antenna and the LNA. The LNA is designed to provide a high gain while contributing a minimal amount of excess noise beyond the noise appearing at the LNA input. This property is known as a low noise figure. To achieve a high linearity characteristic, the LNA should also have a high third-order input intercept point (IIP3). IIP3 is the input level where the third-order intermodulation product levels equal the extrapolated linear desired output level. In general, a high value of IIP3 indicates high linearity performance.
One way to achieve high linearity in a receiver is to incorporate a high performance LNA design which includes a degeneration inductor switching scheme at the LNA input to implement a low linearity (LL) mode, a middle linearity (ML) mode or a high linearity (HL) mode. In one aspect, low linearity corresponds to low noise. The addition of degeneration inductors in the LNA design improves device input/output impedance matching, noise matching, stability and linearity but with reduced gain. However, in the high frequency especially 2 GHz and above frequency range (including International Mobile Telecommunications (IMT) and higher frequency bands), the LNA designs with switching degeneration inductor schemes have degraded noise performance (i.e., increased noise figure) compared to their performance at lower frequencies.
Typical receivers also incorporate a downconverter mixer after the LNA to convert the receiver radio frequency (RF) band to an intermediate frequency (IF) band. Downconversion is performed because subsequent signal processing, such as, gain control, bandpass filtering, etc., is more conveniently employed in the IF band. In some receivers, a bandpass filter is included prior to the downconverter mixer to eliminate undesired products (such as transmit-to-receive noise leakage) from the frequency downconversion. One example of a bandpass filter used in receiver designs is a surface acoustic wave (SAW) filter. However, the bandpass filter adds insertion loss and mass to the receiver design. Thus, the vulnerability of the receiver to undesired products such as transmit-to-receive noise leakage is mitigated by the addition of a bandpass filter at the expense of added insertion loss and mass, which requires better noise performance on the LNA to compensate for the worse SNR caused by the additional insertion loss.